The invention relates to optical fiber devices and methods for manipulating optical signal strength and systems using such devices and methods. More particularly, the invention relates to all-fiber optical devices, methods, and systems for modifying propagation properties of optical information.
Optical devices that modify the properties of optical signals include devices such as modulators and attenuators. Such devices use various means to vary refractive properties of one or more regions of the device to change the amplitude/phase of a signal propagating through the device. Conventional devices of this kind use lithium niobate, electroabsorption and/or other configurations to affect the propagation properties of an optical fiber or other waveguide arrangement. Typically, conventional modulators and similar devices are relatively costly and bulky devices that introduce an undesirable amount of loss. However, conventional all-fiber modulators attempt to establish compatibility with other fibers and provide lower insertion loss and relatively compact size.
Many conventional all-fiber modulators and attenuators include devices based on fiber arrangements having a segment of the core vulnerable to an electro-optic polymer. Such devices include, e.g., D-fiber, manufactured by KVH Industries, Inc. See, e.g., U.S. Pat. Nos. 5,768,462, 6,041,149, and 6,134,356, in which one or more grooves for positioning electrodes therein are formed in the protective (buffer) layer surrounding the core and cladding regions of the optical fiber. However, the configuration of such devices makes them relatively difficult to manufacture. Also, such devices are asymmetric, with undesirable associated polarization properties.
Other conventional modulating devices, e.g., variable optical attenuators, use micro-electromechnical systems (MEMS) technology, which often is unreliable. Also, many conventional attenuators use planar waveguide technology and/or polymers. However, such conventional devices are relatively large in size, introduce a relatively large amount of loss to an optical system, and exhibit relatively significant polarization dependence.
Accordingly, it is desirable to have available an all-fiber, optical device such as an optical modulator or variable optical attenuator that has greater performance efficiency, easier manufacturability, and generally is smaller in size than conventional modulators and attenuators.
The invention is embodied in an optical fiber device such as a modulator, variable optical attenuator or tunable filter, a system for use therein, and method for making the optical device. The optical device includes a length of optical fiber having a core region and a cladding layer formed around the core region. The cladding layer includes controllable active material disposed therein, e.g., in capillaries, pockets or rings formed in the cladding layer. The active materials include, e.g., electro-optic material, magneto-optic material, photorefractive material, thermo-optic material, and/or materials that provide tunable gain or loss (e.g., laser dyes or composite materials such as polymers dispersed with erbium particles). The application of, e.g., temperature, light (optical field) or an electric or magnetic field varies optical properties such as refractive index, loss, scattering, or birefringance of the active material, which, in turn, varies or affects the propagation properties of optical signals in the device.
The optical device also includes a tapered region that reduces the diameter of the fiber device but maintains the relative dimensional proportions as in the non-tapered regions. In the tapered region, the mode field is not supported by the doped core and spreads into the cladding region, where it interacts with the active materials. Simultaneously, the tapered region allows the active material to be physically closer to the propagated modes compared to conventional arrangements, thus allowing interaction between the active material and the propagating modes. The tapered region also is designed such that the fiber easily can be spliced to conventional fiber.